Maarten Krabbendam

Sedimentary basin and detrital zircon record along East Laurentia and Baltica during assembly and breakup of Rodinia

Tectonic processes associated with supercontinent cycles result in a variety of basin types, and the isotopic dating of detrital minerals within sedimentary sequences assists palaeogeographical reconstructions. Basins located along the Laurentia–Baltica margin prior to assembly of Rodinia at 1.2–1.0 Ga are dominated by zircon detritus derived from contemporaneous magmatic arcs. Basins formed during assembly are also dominated by zircon detritus with ages similar to that of sediment accumulation, reflecting syn-collisional magmatism and rapid exhumation of the developing Grenville–Sveconorwegian orogen. Post-collision intracratonic basins lack input from syn-depositional magmatism, and are dominated by significantly older detritus derived from the mountain range as well as its foreland. Basins formed during late Neoproterozoic to Cambrian breakup of Rodinia are divisible into two types. Those within the Caledonides lie on the Grenville–Sveconorwegian foreland and incorporate Archaean and Palaeoproterozoic detritus derived from the cratonic interior and Mesoproterozoic detritus derived from the eroded remnants of the orogen. In the Appalachian orogen, such basins are dominated by Mesoproterozoic detritus with older detritus forming only a minor component, suggesting restricted input from the cratonic interior as a result of either the Grenville orogen still forming a drainage divide or the formation of rift shoulders.

Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledonides

Abstract In the Western Gneiss Region (WGR) in the Scandinavian Caledonides, Scandian ecologites (P = 16 to >28 kbar) occur in a large area of reworked Proterozoic gneisses, structurally below a series of Scandian nappes. The top-to-the-west, extensional Nordfjord-Sogn Detachment (NSD) separates the WGR from allochthonous units, which include several late-orogenic Devonian basins. The allochthon has not experienced Scandian high-pressure (HP) metamorphism. Below the NSD, the WGR is intensely deformed under late-orogenic amphibolite-facies conditions. This deformation is bulk-constrictional, as indicated by a linear feldspar fabric within augen gneisses and tight to isoclinal, lineation-parallel folds within layered gneisses. In a later stage, the NSD, the WGR and the Devonian basins were folded by east-west trending folds, coeval with continuing movement along detachments. To explain these features we propose a transtensional model for the late-orogenic evolution of the WGR. Transtension in West Norway had a sinistral sense and was partially partitioned with increasing transtensional angle towards the NE-SW trending Møre-Trøndelag Fault Zone in the NW. During transtension, there is a strong tendency for rejuvenation of detachments, because detachments fold and may lock as they move. In the WGR, the younger Hornelen Detachment developed above the older NSD. Transtension was the principal exhumation mechanism of the HP and ultra-high-pressure (UHP) rocks in the WGR and involved oblique plate divergence of Laurentia and Baltica during the Early Devonian.

A fluvial origin for the Neoproterozoic Morar Group, NW Scotland; implications for Torridon–Morar Group correlation and the Grenville Orogen foreland basin

Precambrian sedimentary successions are difficult to date and correlate. In the Scottish Highlands, potential correlations between the thick, undeformed siliciclastic ‘Torridonian’ successions in the foreland of the Caledonian Orogen and the highly deformed and metamorphosed siliciclastic Moine succession within the Caledonian Orogen have long intrigued geologists. New and detailed mapping of the Neoproterozoic Altnaharra Formation (Morar Group, lowest Moine Supergroup) in Sutherland has discovered low-strain zones exhibiting well-preserved sedimentary features. The formation comprises 3–5 km of coarse, thick-bedded psammite with abundant nested trough and planar cross-bedding bedforms, defining metre-scale channels. Palaeocurrent directions are broadly unimodal to the NNE–ENE. We interpret the Altnaharra Formation as high-energy, braided fluvial deposits. The Altnaharra Formation and the unmetamorphosed, Neoproterozoic Applecross–Aultbea formations (Torridon Group) are similar in terms of lithology, stratigraphical thickness, sedimentology, geochemistry, detrital zircon ages and stratigraphical position on Archaean basement. Depositional age constraints for both successions overlap and are coeval with late Grenvillean orogenic activity. Detrital zircons imply similar source regions from the Grenville Orogen. The Morar and Torridon groups can thus be correlated across the Caledonian Moine Thrust and are best explained as parts of a single, large-scale, orogen-parallel foreland basin to the Grenville Orogen.

Continental tectonics and mountain building : the legacy of Peach and Horne

The worlds mountain ranges are the clearest manifestations of long-term deformation of the continental crust. As such they have attracted geological investigations for centuries. Throughout this long history of research a few keynote publications stand out. One of the most important is the Geological Surveys 1907 Memoir on The Geological Structure of the North-West Highlands of Scotland . The Memoir summarized some of the Geological Surveys finest work, and outlined many of the principles of field-based structural and tectonic analysis that have subsequently guided generations of geologists working in other mountain belts, both ancient and modern. The thematic set of 32 papers in this Special Publication celebrate the 100th anniversary of the 1907 Memoir by placing the original findings in both historical and modern contexts, and juxtaposing them against present-day studies of deformation processes operating not only in the NW Highlands, but also in other mountain belts.

Grain size stabilisation by dispersed graphite in a high-grade quartz mylonite: an example from Naxos (Greece)

High grade quartz mylonites from Naxos, Greece, consist of alternating thin layers of pure quartz and quartz layers with 0.3– 3 vol.% finely dispersed graphite particles. Graphite-free layers are coarse grained (100 – 300 mm), show undulose extinction, subgrains, lobate grain boundaries and have a strongly developed crystallographic preferred orientation. In these layers, dislocation flow is interpreted to be the dominant deformation mechanism. In contrast, graphite-rich layers are fine-grained (30 – 70 mm), have equant quartz grain shapes and have a crystallographic preferred orientation that becomes progressively weaker with decreasing quartz grain size. Given the high temperature of deformation and the need for a c-axis fabric destroying mechanism, grain boundary sliding is interpreted to be important in these layers. Analysis shows an inverse relationship between quartz grain size and the graphite dispersion, suggesting stabilization of quartz grain size by graphite particles. Graphite particles larger than 5 mm are concentrated along quartz boundaries, suggesting that stabilisation only operates above a certain critical graphite particle size. This study shows that a dispersed second phase such as graphite in a naturally deforming rock can inhibit grain boundary migration, stabilise the grain size and enhance grain boundary sliding at the expense of dislocation flow. q 2002 Elsevier Science Ltd. All rights reserved.

Timing of regional deformation and development of the Moine Thrust Zone in the Scottish Caledonides: constraints from the U–Pb geochronology of alkaline intrusions

Abstract: The Moine Thrust Zone in the Scottish Highlands developed during the Scandian Event of the Caledonian Orogeny, and now forms the boundary between the Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the formation of the Moine Thrust Zone, have previously been dated by a range of isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions localized along the thrust zone have provided the best age constraints for deformation. Recent British Geological Survey mapping has improved our understanding of the structural relationships of some of these intrusions, and this work is combined with new U–Pb dates in this paper to provide significantly improved ages for the Moine Thrust Zone. Our work shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within the orogenic belt to the east of the Moine Thrust Zone at 447.9 ± 2.9 Ma. A more significant pulse of magmatism centred in the Assynt area, which temporally overlapped movement in the thrust zone, occurred at 430.7 ± 0.5 Ma. Movement in the thrust zone had largely ceased by the time of emplacement of the youngest intrusions, the late suite of the Loch Borralan Pluton, at 429.2 ± 0.5 Ma, and the Loch Loyal Syenite Complex.

The Moine Supergroup of NW Scotland: insights into the analysis of polyorogenic supracrustal sequences

Abstract The Moine Supergroup of NW Scotland is a thick sequence of early Neoproterozoic sedimentary rocks, with minor igneous intrusions, that display evidence for multiple phases of regional deformation and metamorphism. The descriptions and interpretations of the ‘Moine Schists’ provided by the 1907 memoir (Peach et al. 1907) have been proved to be essentially correct and have laid the groundwork for a century of distinguished and influential research that has reached far beyond the confines of NW Scotland. The Survey workers recognized the sedimentary protoliths of these rocks, realized that they had been deposited unconformably on inliers of reworked basement gneisses that now occupy the cores of major folds, and understood the likely complexity of folding and the kinematic significance of mineral lineations. Further advances in understanding of the Moine rocks were mainly achieved through two techniques that were not available to the Survey workers of 100 years ago – geochronology and palaeomagnetism. Isotopic studies have confirmed the view of the Survey workers that the Moine rocks are of Precambrian age, and furthermore have demonstrated a complex, polyorogenic history.

The Laxford Shear Zone: an end-Archaean terrane boundary?

Abstract The Lewisian Gneiss Complex of northwestern Scotland consists of Archaean gneisses, variably reworked during the Proterozoic. It can be divided into three districts – a central granulite-facies district between districts of amphibolite-facies gneiss to the north and south. Recent work has interpreted these districts in terms of separate terranes, initiating a controversy that has implications for how Precambrian rocks are understood worldwide. The northern district of the Lewisian Gneiss Complex (the Rhiconich terrane) is separated from the central district (the Assynt terrane) by a broad ductile shear zone known as the Laxford Shear Zone. This paper reviews the geology of the Laxford Shear Zone, clarifying field relationships and discussing other evidence, to consider whether or not it does indeed represent a terrane boundary. A detailed review of field, geochemical and geochronological evidence supports the recognition of the separate Assynt and Rhiconich terranes. Mafic dykes (the Scourie Dyke Swarm) and granitoids, of Palaeoproterozoic age, occur on both sides of the Laxford Shear Zone and thus the terranes were most probably juxtaposed during the late Archaean to early Palaeoproterozoic Inverian event. Subsequently, the less-competent, more-hydrous amphibolite-facies gneisses of the Rhiconich terrane were affected by later Palaeoproterozoic (Laxfordian) deformation and partial melting, to a greater extent than the more-competent granulite-facies gneisses of the Assynt terrane.

The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs – Beinn Dearg area, NW Highlands

Synopsis The Morar Group, the lowest group of the early Neoproterozoic Moine Supergroup in the Scottish Highlands, forms a >5 km thick metamorphosed siliclastic sequence, recently interpreted to form part of a Grenvillian (c. 1000 Ma) foreland basin. New mapping has elucidated the structure and stratigraphy of the Morar Group in the Fannich–Beinn Dearg area, where the Morar Group occurs in a single coherent thrust sheet (Achness Thrust Sheet), over 70 km long, 20 km wide, and up to 10 km thick. Within this thrust sheet, the strata are folded by two very large, west-vergent and west-facing cylindroidal anticline-syncline pairs that deform the overlying Sgurr Beag Thrust. The lowest long limb is parallel with and grades into the ductile Moine Thrust and Achness Thrust at its base. Low strain zones in steep limbs contain well preserved sedimentary structures. Reconstruction of the stratigraphical architecture shows five formations of metasandstone (psammite), alternating with meta-siltstone (semipelite). Large-scale lateral variations in the lowest metasandstone package are capped by a possible flooding surface of semipelite, followed by more metasandstone. The deformation history shows foreland-propagation of both deformation and metamorphism, from NNW-directed transport on the Sgurr Beag Thrust to WNW-directed transport on the Achness Thrust and Moine Thrust.

Progressive fold and fabric evolution associated with regional strain gradients: a case study from across a Scandian ductile thrust nappe, Scottish Caledonides

Abstract Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D2 strain towards the basal ductile detachment. Within the upper greenschist to lower amphibolite facies thrust sheet, the main gently east-dipping foliations and SE-plunging transport-parallel lineations maintain a broadly similar orientation over c. 600 km2. Associated main phase, thrust-related folds (F2) are widely developed, and towards the base of the thrust sheet display progressive tightening and increasing curvilinearity of fold hinges ultimately resulting in sheath folds. Secondary folds (F3) are largely restricted to high-strain zones and are interpreted as flow perturbation folds formed during non-coaxial, top-to-the-NW ductile thrusting. These features are consistent with a structural model that incorporates plane strain pure-shear flattening with a superimposed and highly variable simple shear component focused into high-strain zones. The increase in strain over a distance of 30 km across strike is similar to the increasing deformation observed when structures are traced along strike to the north, and which are apparently related to proximity to basement-cover contacts. A U–Pb zircon age of 415±6 Ma obtained from a syn-D2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.